1. Field of the Invention
The present invention relates to an aberration corrector for use in an instrument (e.g., an instrument using an electron beam or ion beam, such as a scanning electron microscope or ion microprobe) utilizing a charged-particle beam to correct chromatic and spherical aberrations in such an instrument.
2. Description of Related Art
In a scanning electron microscope or transmission electron microscope, an aberration corrector is incorporated in the optics in order to provide high-resolution imaging or enhance the probe current density. One proposed example of this aberration corrector uses a combination of electrostatic quadrupole elements and magnetic quadrupole elements to correct chromatic aberration. The corrector also uses four stages of octupole elements to correct spherical aberration. The principle is introduced in detail in various literature: [1] H. Rose, Optik 33, Heft 1, pp. 1-24 (1971); [2] J. Zach, Optik 83, No. 1, pp. 30-40 (1989); and [3] J. Zach and M. Haider, Nucl. Instru. and Meth. in Phys. Res. A 363, pp. 316-325 (1995).
The principle of the above-described aberration corrector is described briefly now by referring to FIG. 1, where an aberration corrector C is placed ahead of an objective lens 7. The aberration corrector C comprises four stages of electrostatic quadrupole elements 1, 2, 3, 4, two stages of magnetic quadrupole elements 5, 6, and four stages of electrostatic octupole elements 11, 12, 13, 14. The two stages of magnetic quadrupole elements 5, 6 create a magnetic potential distribution analogous to the electric potential distribution created by the second and third stages of the electrostatic quadrupole elements to produce a magnetic field superimposed on the electric field. The four stages of electrostatic octupole elements 11, 12, 13, 14 create an electric field superimposed on the electric field created by the four stages of electrostatic quadrupole elements 1-4.
In an actual instrument, four stages of dipole elements and four stages of hexapole elements are also mounted to produce fields superimposed on the fields created by the aforementioned quadrupole and octupole elements. The dipole elements act as deflecting devices for axial alignment. The hexapole elements act to correct the second-order aperture aberration. Since these dipole and hexapole elements are not closely related to the present invention, they will not be described in detail below.
In this configuration, a beam of charged particles is entered from the left side as viewed in the figure. The four stages of electrostatic quadrupole elements 1-4 and the objective lens 7 together act to form a reference orbit for the beam. As a result, the beam is focused onto a specimen surface 20. In FIG. 1, both orbit Rx of the particle beam in the X-direction and orbit Ry in the Y-direction are schematically drawn on the same plane.
The reference orbit can be regarded as a paraxial orbit, that is, an orbit assumed where there is no aberration. The quadrupole element 1 causes the Y-direction orbit Ry to pass through the center of the quadrupole element 2. The quadrupole element 2 causes the X-direction orbit Rx to pass through the center of the quadrupole element 3. Finally, the quadrupole elements 3, 4 and objective lens 7 together focus the beam onto the specimen surface. In practice, these components need to be adjusted mutually for complete focusing. At this time, the four stages of dipole elements are used for axial alignment.
Referring more particularly to FIG. 1, the charged-particle beam in the X-direction orbit Rx is diverged by the quadrupole element 1 acting like a concave lens. Then, the beam is converged by the quadrupole element 2 acting like a convex lens. The beam is thus made to pass through the center of the quadrupole element 3. Then, the beam is converged by the quadrupole element 4 and travels toward the objective lens 7. On the other hand, the charged-particle beam in the Y-direction orbit Ry is converged by the quadrupole element 1 and made to pass through the center of the quadrupole element 2. Then, the beam is converged by the quadrupole element 3. Finally, the beam is diverged by the quadrupole element 4 and moves toward the objective lens 7. In this way, the function of a single concave lens is created by combining the divergent action of the quadrupole element 1 acting on the X-direction orbit Rx and the divergent action of the quadrupole element 4 acting on the Y-direction orbit Ry.
Correction of chromatic aberration using the aberration corrector C is described. To correct chromatic aberration by the system shown in FIG. 1, the potential xcfx86q2 volts at the electrostatic quadrupole element 2 and the magnetic excitation J2 amp turns (or magnetic potential) of the magnetic quadrupole element 5 are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, the potential xcfx86q3 volts at the electrostatic quadrupole element 3 and the magnetic excitation J3 amp turns of the magnetic quadrupole element 6 are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the Y-direction chromatic aberration to zero.
Correction of spherical aberration (correction of the third-order aperture aberration) is next described. Before spherical aberration is corrected, X- and Y-direction chromatic aberrations are corrected. Then, the X-direction spherical aberration in the whole lens system is corrected to zero by the potential xcfx8602 volts at the electrostatic octupole element 12. The Y-direction spherical aberration is corrected to zero by the potential xcfx8603 volts at the electrostatic octupole element 13.
Then, the spherical aberration in the resultant direction of the X- and Y-directions is corrected to zero by the electrostatic octupole elements 11 and 14. In practice, repeated mutual adjustments are necessary. Superimposition of the potentials and magnetic excitations at the quadrupole and octupole elements has been put into practical use by varying the potential or excitation applied to each pole of a single twelve-pole element by using this twelve-pole element to synthesize dipoles, quadrupoles, hexapoles, octupoles, etc. This method has been introduced, for example, in [4] M. Haider et al., Optik 63, No. 1, pp. 9-23 (1982).
In particular, in an electrostatic design, a final stage of power supplies An (n=1, 2, . . . , 12) capable of supplying a voltage to 12 electrodes Un (n=1, 2, . . . , 12) independently is connected as shown in FIG. 9 of this patent application. Where a quadrupole field is produced, output voltages from a quadrupole power supply 10 are supplied to the final-stage power supplies An to obtain a quadrupole field close to an ideal quadrupole field. If it is assumed that the output voltages from the final-stage power supplies An are proportional to the output voltages from the quadrupole power supply 10, the ratio of the output voltages from the power supply 10 assumes a value as given in the reference [4] above. Where an octupole field is created to be superimposed on this quadrupole field, output voltages from an octupole power supply 18 are added to the output voltages from the quadrupole power supply 10 and supplied to the final-stage power supplies An to obtain a field close to an ideal octupole field. Similarly, a field on which a multipole field produced by a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained using the single twelve-pole element.
In a magnetic design, a final stage of power supplies Bn (n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on 12 magnets Wn (n=1, 2, . . . , 12) independently is connected as shown in FIG. 10 of this patent application. Where a quadrupole magnetic field is created, output voltages from a quadrupole magnetic-field power supply 15 are supplied to the final stage of power supplies Bn to produce a field close to an ideal quadrupole magnetic field. If it is assumed that the output currents from the final-stage power supplies Bn are proportional to the output voltage from the quadrupole magnetic-field power supply 15, the ratio of the output voltages from the power supply 15 assumes a magnetic exciting ratio as given in the reference [4] above. Superimposition of multipole fields other than a quadrupole magnetic field is not explained herein. However, multipole fields other than a quadrupole magnetic field can be superimposed in the same way as in the electrostatic design, by adding voltages for multipole fields to the input voltage to the final-stage power supplies Bn. A yoke for magnetically connecting the outside portions of the magnets Wn is omitted in FIG. 10.
Where electrostatic and magnetic designs are superimposed, a conductive magnetic material may be used so that the magnets Wn can act also as the electrodes Un. In this case, the coils on the magnets are mounted so as to be electrically isolated from the electrodes.
In the description given below, the 2n-pole elements are treated as if they were superimposed on top of each other to simplify the explanation. In practice, superimposition of multipole fields on a single twelve-pole field is achieved by adding voltage signals as mentioned previously.
After correction of chromatic aberration, it may be necessary to correct the second-order aperture aberration by means of four stages of hexapole elements before correction of spherical aberration is performed. This correction is made in the same procedure as in the aforementioned correction of spherical aberration. This second-order aperture aberration occurs depending on the mechanical accuracy of the aberration corrector. Normally, the amount of correction is small, and this aberration affects higher-order aberrations only a little within the scope of the present invention. The second-order aperture aberration is corrected within the aberration corrector. If the resultant magnification (described later) of the aberration corrector and the objective lens is varied, higher-order aberrations are affected little, though the resultant magnification is important in the present invention. Therefore, description of the correction of the second-order aperture aberration is omitted herein.
Potential or voltage xcfx86 used in the following description regarding electrostatic multipole elements indicates a positive value of the multipole elements arranged normally as shown in FIGS. 2(a) and 2(b). Similarly, magnetic excitation J of magnetic type indicates magnetic excitation amp turns on the positive side.
The aforementioned theory of aberration correction and the results of actually performed experiments demonstrate that chromatic and spherical aberrations are almost completely corrected. This proves the excellence of the aberration correction system described above. From a practical point of view, it can be said that sufficient consideration has not been given to the stability of the aberration correction system and to the range of the applied voltage. Therefore, the following problems have arisen.
First, where the accelerating voltage Va for a particle probe or the working distance WD indicative of the space between the objective lens 7 and the specimen surface 20 is varied, the magnetic excitation of the magnetic quadrupole is correspondingly varied. Concomitantly, slight magnetic drift xcex4B occurs. Where long-term, high stability is required, this drift may shift the reference orbit for the charged-particle beam and disturb the aberration correction conditions.
Secondly, where the accelerating voltage for the charged-particle beam is low, if the voltage necessary for correction of chromatic and spherical aberrations is set to a higher voltage in some degree, the voltage resistance (or withstand voltage) of the electrostatic multipole elements, such as quadrupole and octupole elements, presents no problems. We have found that where the accelerating voltage is increased, the magnitude of the correcting voltage greatly exceeds the voltage-resistance limit of the multipole elements.
Thirdly, where the accelerating voltage is used over a wide range from low to high voltage, the voltage of the whole system is low at low accelerating voltages. The voltage applied to each electrostatic quadrupole element contains fluctuation component xcex4xcfx86. At lower voltages, a greater portion of this fluctuation component is given to the correcting voltage and deflecting field. Therefore, where long-term stability or high resolution is required, this fluctuation component xcex4xcfx86 presents problems.
Fourthly, certain conditions under which the effects of higher-order aberrations remaining left after correction of chromatic and spherical aberrations are minimized have not been taken into consideration. Examples of these higher-order aberrations include fifth-order aperture aberration (i.e., the amount of aberration is in proportion to the fifth power of the angular aperture of the beam impinging on the specimen surface) and the fourth-order aperture and chromatic aberration (i.e., the amount of aberration is in proportion to the product of the third power of the angular aperture of the beam and the energy spread of the beam, namely, the combined aberration of third-order aperture aberration and first-order chromatic aberration).
In view of the foregoing circumstances, the present invention has been made. It is an object of the present invention to realize an aberration corrector which is for use in an instrument utilizing a charged-particle beam and which can make optimum aberration correction stably over a long term.
An aberration corrector built in accordance with the present invention is incorporated within the optics of an instrument utilizing a charged-particle beam and comprises four stages of electrostatic quadrupole elements, including two central stages of quadrupole elements, two stages of magnetic quadrupole elements for superimposing a magnetic potential distribution analogous to an electric potential distribution created by the two central stages of quadrupole elements on the electric potential distribution, an objective lens for focusing the charged-particle beam onto a specimen, an objective aperture placed in a part of an optical path for the charged-particle beam, a manual operation portion permitting one to modify the accelerating voltage for the charged-particle beam or the working distance between the objective lens and the specimen, and a control portion for controlling the quadrupole elements and the objective lens according to a manual operation or setting performed on the manual operation portion.
In one feature of the present invention, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the accelerating voltage is varied under conditions where the excitations of the magnetic quadrupole elements are maintained constant and the control portion adjusts (2) the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the working distance is varied under conditions where the excitations of the magnetic quadrupole elements are maintained constant, whereby chromatic aberration in the optics for the charged-particle beam is corrected.
In another feature of the present invention, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and electrical currents for exciting the magnetic quadrupole elements while maintaining constant the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the accelerating voltage is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform and the control portion adjusts (2) the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the working distance is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform, whereby chromatic aberration in the optics for the charged-particle beam is corrected.
In another feature of the present invention, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and electrical currents for exciting the magnetic quadrupole elements while maintaining constant the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the accelerating voltage is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform and the control portion adjusts (2) the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the working distance is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform to thereby minimize the absolute value of the fifth-order aperture aberration or the fourth-order aperture aberration (i.e., combined aberration of third-order aperture aberration and first-order chromatic aberration) is reduced to zero or its absolute value is reduced to a minimum.
In an additional feature of the present invention, there are further provided four stages of electrostatic octupole elements for superimposing an octupole electric potential on the electric potential distribution created by the four stages of electrostatic quadrupole elements and another control portion for controlling the four stages of electrostatic octupole elements according to a manual operation or setting performed on the manual operation portion, whereby spherical aberration can also be corrected.
In still another feature of the present invention, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and the electrical currents for exciting the magnetic quadrupole elements while maintaining constant the resultant magnification of the fourth stage of electrostatic quadrupole elements and the objective lens if the accelerating voltage is varied under conditions where the absolute values of sums of the voltages across the four stages of electrostatic quadrupole elements and voltages across the electrostatic octupole elements respectively superimposed on the first-mentioned voltages are substantially uniform and the control portion adjusts (2) the resultant magnification of the fourth stage of electrostatic quadrupole element and the objective lens if the working distance is varied under conditions where the absolute values of sums of the voltages across the four stages of electrostatic quadrupole elements and voltages across the electrostatic octupole elements respectively superimposed on the first-mentioned voltages are substantially uniform to thereby correct at least one of chromatic and spherical aberrations in the optics for the charged-particle beam.
The present invention also provides an aberration corrector for use in an instrument utilizing a charged-particle beam, the aberration corrector being incorporated in the optics of the instrument, the aberration corrector comprising: four stages of electrostatic quadrupole elements including two central stages of quadrupole elements; two stages of magnetic quadrupole elements for superimposing a magnetic potential distribution analogous to an electric potential distribution created by the two central stages of quadrupole elements on the electric potential distribution; an objective lens for focusing the charged-particle beam onto a specimen; an objective aperture placed in a part of an optical path for the charged-particle beam; an additional lens mounted between a quadrupole element assembly formed by the four stages of electrostatic quadrupole elements and the objective lens; a manual operation portion permitting one to modify the accelerating voltage for the charged-particle beam or the working distance between the objective lens and the specimen; and a control portion for controlling the four stages of electrostatic quadrupole elements, the two stages of magnetic quadrupole elements, the objective lens, and the additional lens according to a manual operation or setting performed on the manual operation portion.
In one feature of this aberration corrector, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and the resultant magnification of the additional lens and the objective lens if the accelerating voltage is varied under conditions where the excitations of the magnetic quadrupole elements are maintained constant and the control portion adjusts (2) the resultant magnification of the additional lens and the objective lens if the working distance is varied under conditions where the excitations of the magnetic quadrupole elements are maintained constant, to thereby correct chromatic aberration in the optics for the charged-particle beam.
In another feature of this aberration corrector, when either the accelerating voltage or the working distance is varied according to a manual operation or setting performed on the manual operation portion, the control portion adjusts (1) the voltages applied to the electrostatic quadrupole elements and electrical currents for exciting the magnetic quadrupole elements while maintaining constant the resultant magnification of the fourth stage of the additional lens and the objective lens if the accelerating voltage is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform and the control portion adjusts (2) the resultant magnification of the additional lens and the objective lens if the working distance is varied under conditions where the absolute values of the voltages across the four stages of electrostatic quadrupole elements are substantially uniform to thereby correct chromatic aberration in the optics for the charged-particle beam.
In a further feature of this aberration corrector, the control portion sets the electric potentials at the two central stages of electrostatic quadrupole elements for correcting chromatic aberration and the electric potentials at the four stages of electrostatic octupole elements to their preset values, whereby the absolute value of the fifth-order aperture aberration is minimized or the fourth-order combined aperture and chromatic aberration (i.e., combined aberration of third-order aperture aberration and first-order chromatic aberration) is reduced to zero or its absolute value is reduced to a minimum.
Other objects and features of the invention will appear in the course of the description thereof, which follows.